1. Field of the Invention
This invention relates to zoom lenses of the rear focus type and, more particularly, to zoom lenses of the rear focus type suited to extend the zooming range up to 6 and increase the aperture ratio to about 1.8 in F-number for use in video cameras, still video cameras or cameras for broadcasting. Still more particularly, it relates to improvements over U.S. Pat. No. 4,859,042 and U.S. Pat. No. 5,009,492 commonly assigned to the assignee of the present invention.
2. Description of the Related Art
A type of zoom lens in which a lens unit other than the front or first lens unit is used for focusing, or the so-called rear focus type, has been employed in many previous proposals for zoom lenses in cameras for photography, video cameras, etc.
In general, the use of the rear focus type in zoom lenses decreases the effective diameter of the first lens unit as compared with a zoom lens in which a first lens unit is moved for focusing, making it easier to construct the entire lens system in compact form, and also to extend the focusing range toward shorter object distances, particularly to make close-up photography. Still another advantage arising from a relatively small size and light weight the focusing lens unit has is that the power of driving the focusing lens unit may be decreased, so that speedy focusing can be done at the ordinary power.
Such a zoom lens of the rear focus type is disclosed in, for example, Japanese Laid-Open Patent Application No. Sho 63-44614, comprising, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of negative refractive power for compensating for the image shift as the focal length varies, and a fourth lens unit of positive refractive power, these four lens units in total, or a so-called 4-unit zoom lens, wherein the third lens unit is made movable for focusing. However, this zoom lens has to get a space large enough to assure the total movement of the third lens unit. This leads to a tendency to increase the total lens length.
In Japanese Laid-Open Patent Application No. Sho 58-136012, the zooming section is constructed with three or more lens units, part of which is moved to effect focusing.
In Japanese Laid-Open Patent Application No. Sho 63-247316, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, wherein the second lens unit is moved to vary the focal length, and the fourth lens unit is moved to compensate for the image shift as the focal length varies and to adjust focus.
In Japanese Laid-Open Patent Application No. Sho 58-160913, of the four lens units of positive, negative, positive and positive refractive powers in this order from the object side, the first and second lens units are moved to vary the focal length, and the fourth lens unit is moved to compensate for the image shift as the focal length varies. Further, one or two of these lens units are made movable for focusing.
The zoom lenses of the rear focus type generally have a minimized size and an ability to perform speedy focusing, and are feasible to nearer close-up photography.
On the other hand, however, the range of fluctuation of aberrations at the time of focusing is caused to increase largely. Therefore, a problem arises that it becomes very difficult to maintain good stability of optical performance at a high level throughout the entire range of object distances from an infinitely distant object to a closest object, while still maintaining the minimization of the bulk and size of the entire lens system.
Particularly when the aperture ratio and zoom ratio of the zoom lens are increased at once, an additional problem arises that it becomes very difficult to obtain a high optical performance throughout the extended zooming range as well as throughout the entire focusing range.
Meanwhile, since, in the rear focus type of zoom lens, the focusing movement of the focusing lens varies depending on the focal length of the entire lens system, the auto-focus techniques of the external measurement type using infrared light or the like are hardly applicable.
So, the common method of automatic focusing used in video cameras or the like is to use the video signals.
In this method, either the image sensor or part of the lens system is subjected to minute vibration so as to obtain a direction discriminating signal for automatic focusing. This method is very advantageous for improving the accuracy of focusing control and the focusing speed.
If it is the image sensor such as a CCD that is subjected to minute vibration by using the piezoelectric element, the advantage lies in the point that the signal is obtained in no relation with the position sensitivity of the focusing lens. However, this technique means the necessity of surplus driving means. So, in this point, the complexity of structure is caused to increase, being disadvantageous in terms of the cost. As the zoom lens is of the rear focus type, particularly with the use of the lens unit nearest to the image plane in focusing, the sensitivity of the focusing lens varies to a relatively small extent. Hence, it is advantageous to subject the focusing lens to minute vibration, or so-called wobbling, so as to obtain the signal representing the direction to which automatic focusing is to go, so that the structure is simplified and the cost is reduced.
When the focusing lens is subjected to minute vibration, the point at which the off-axial light ray focuses itself changes in position, or the so-called image shake phenomenon takes place.